2,2′-bipyridine) triplet excited state by transition-metal coordination compounds has long been established and often proceeds via energy transfer
(25–28); however, this phenomenon has not been
utilized in bond-forming catalysis. We noted that
an analogous role-separation photosensitization
concept might indeed be applied to organometallic
catalysis. More specifically, using a strongly absorbing chromophore as a catalyst, we were able
to gain entry to organometallic catalysts in their
excited states via energy-transfer photosensitization
and, thereafter, successfully access mechanistically
distinct cross-coupling pathways. In this context,
we report here the coupling of carboxylic acids with
aryl halides to generate O-aryl ester products using
a dtbbpy·Ni(0) complex (dtbbpy is 4,4′-di-tert-
butyl-2,2′-bipyridine) as the cross-coupling catalyst
and Ir(ppy)3 (1) (where ppy is ortho-metalated
2-phenylpyridine) as the optimal photosensitizer.
Despite an extensive body of research into C–
heteroatom bond formation over the past 40 years,
an enduring challenge in this field has been the
development of an efficient, mild, catalytic coupling between carboxylic acids and aryl halides
to form O-aryl esters (29–33). As shown in Fig.
2A, our initial studies revealed that treatment of
benzoic acid (3), with methyl p-bromobenzoate
(4), enables the desired coupling in 85% yield
using visible light and the catalyst combination
outlined above. We further found that the organic sensitizer benzophenone (2) is also a moderately efficient photocatalyst in combination
with the nickel catalyst described above (Fig. 2A
and see the supplementary materials). Although
in no way mechanistically definitive, we recognized that the successful use of benzophenone
might implicate an energy transfer pathway in
lieu of a SET process and that the efficiency of
the Ni cycle most likely relied on access to an
excited state of an intermediate Ni species. On
this basis, we undertook a substantive mechanistic investigation that we here reveal is fully
consistent with Ni excited-state catalysis, a mode
of activation that is readily accessed using photosensitized energy transfer (14).

A detailed description of the proposed energy
transfer–driven catalytic cycle is shown in Fig. 2B.
The cycle begins with oxidative addition of an
aryl halide to dtbbpy·Ni(0) (6) to provide aryl–
Ni(II) species 7. We believe that coordination
of a carboxylate nucleophile, such as benzoate,
should be rapid, yielding aryl–Ni(II) carboxylate 8.
Meanwhile, Ir(ppy)3 (1) acts as an antenna, absorbing visible light to produce the characteristic
triplet metal-to-ligand charge transfer (3MLCT)
excited state 1*. At this stage, energy transfer
can occur to produce electronically excited Ni(II)
species 8* while simultaneously regenerating the
ground state of 1. Reductive elimination from 8*
generates the O-aryl ester product 5, regenerating Ni(0) species 6, and thus completing the
catalytic cycle.

Despite the catalytic activity of benzophenonein our initial findings, we could not conclusivelyrule out the possibility of a SET oxidation mech-anism on the basis of this information alone.Given our own experimental mechanistic studies(11) and those of Weix, Hu, and Fu and colleagues(34–36), we believed that all steps before reduc-tive elimination would be facile, and as a corollary,we proposed that this was likely the photocatalyst-mediated step. Although computational studieshave indicated that reductive elimination to forma C–O bond from Ni(II) is endothermic (37), ourprevious studies involving a nickel-catalyzed aryletherification reaction had demonstrated thatSET oxidation to Ni(III) is not only possible butis facile and can readily induce the key reduc-tive elimination step.A critical distinction between a photosensi-tization pathway and a SET process is that photo-sensitization creates an excited state of the Ni(II)species that should be directly accessible viaexcitation with visible light in the absence ofthe photocatalyst. Indeed, as shown in Fig. 2A,the coupling of benzoic acid (3) and methyl p-bromobenzoate (4) using our catalytic conditionsbut without a photocatalyst generated the desiredadduct 5 in 45% yield after 120 hours. Further-more, control experiments under identical condi-tions, with the exclusion of light, did not produceany detectable coupling product in the same timeperiod. These experiments are fully consistentwith the formation of a Ni excited-state com-plex that is an on-cycle intermediate and essentialfor productive bond formation.We next turned our attention to studying thenature of the reactivity of the arylnickel(II) carbox-ylate complex subsequent to photosensitization.In this regard, it is important to underscore thefact that the electronic structure of a Ni(II) co-ordination complex is fundamentally differentfrom those of more commonly used complexesof Ru(II), Ir(III), and Cu(I) insofar as the lowestenergy excited state(s) of a Ni(II) system of thetype we are using will be ligand-field (as opposedto charge-transfer) in nature. This class of excitedstates is far more likely to engage in bond-breaking and bond-forming reactions than theelectron transfer chemistry typically observed forcharge-transfer states. This point notwithstand-ing, a mechanistic assessment must still be madeexperimentally. Although time-resolved absorp-tion spectroscopy can, in principle, differentiatebetween energy and electron transfer processes(14), the fact that the Ir sensitizer and the Ni(II)species both absorb in the same region, coupledwith our desire to probe the reaction undercatalytically relevant conditions, rendered mech-anistic determination via this route very challeng-ing (38). As an alternative, we designed a seriesof experiments to probe the capacity of photo-catalysts to perform either energy transfer orelectron transfer in the presence of arylnickel(II)compounds. The Ni(II) complexes are nonemis-sive; although this is consistent with the ex-pected ligand-field nature of the lowest-energyexcited state(s), it prevents a facile direct deter-mination of excited–state energetics. A libraryof heteroleptic iridium photocatalysts of thetype [Ir(ppy)2(ligand)](PF6), where “ligand” refersto a series of symmetrically substituted 2,2′-bipyridine ligands, was therefore synthesized.Systematically varying the electron-donating abil-ity of this ligand allows for controlled tunability